The most common and most likely answer to this crossword clue is the 7 plus 5 letter word DRAGONS TEETH
I've seen this crossword clue in Financial Times
D
R
A
G
O
N
S
T
E
E
T
H
Buddy explains!
The straight part
We know that "They are sown to cause trouble/specifically to tanks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are sown to cause trouble/specifically to tanks = dragons teeth.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.